This project has begun to explore the association between endogenous hormone levels and vocal behavior in the squirrel monkey. In one study, neither cortisol nor testosterone levels correlated with production of alarm calls in adult males. However, a weak correlation was found to exist between cortisol levels and alarm calling rate in adult females. Castrated males were found to have a lower response threshold and steeper dose-response (using rate of alarm calling as the response measure) in a behavioral pharmacology model for defensive threat reactivity. Castrated males also differed from intact males in vocal behavior during same-sex social interactions. The castrates made more twitters and chucks, while intact males made more err chucks and peeps. The former call types are associated with friendly interactions, while the latter are associated with excitement and arousal. New methodology for imaging histological brain sections were initiated, and the brains of adult male and female squirrel monkeys (a species with robust sex differences in behavior) were examined for quantitative differences in the size of forebrain areas.